There is never a convenient time for your water to break, but unless you don't mind cold showers than this is kind of an emergency and you will want to call for a new water heater installation right away.
The actual water heater replacement doesn't take that long, about 3-4 hours and you should be back to business. Do hire a reputable plumber because there are safety standards that need to be complied with for your protection. Your new water heater installation should include earthquake strapping, a safety shut off and a safety overflow that runs off into an area where no one could accidentaly be burned if it overflows. You should have a pan installed underneath as well, in case your water heater ever springs a leak. You should also expect to have your old water heater hauled away.